Three Phase Current, Demand Charges, and Phase Converters

      Shedding some light on ways to minimize power company "demand charges," with an in-depth look at the way phase converters function. December 3, 2010

We have just acquired property to expand our business. Three phase power is available from the power company. Apparently the way the local utility charges is peak for the month, so the minute we start a hard starting machine, like a wide belt sander, that is what we are billed at for the month. It seems that ganging phase converters might be the way to go. Any comments?

Forum Responses
(CNC Forum)
From contributor L:
It is commercial power. Is the single phase priced out different? There is no demand on the single phase when it is in a commercial area. I don't know, just asking. I have 3PH power at my shop and the demand charge is determined by the highest power consumption in a 15 minute segment.

From contributor S:
Does that billing only apply to 3 phase? Just wondering because it takes a lot of amps to start the phase converter to then start the same sander. I would never use a phase converter if I had access to 3 phase power. Phase converters are also hard on machines and there is power loss.

From contributor P:
Are you sure you understand how they are billing for demand electric? Usually a demand meter records the peak amperage total used on any given day. In other words, turn on 3 machines at the same time and you may record 150 amps starting usage, but the three machines run together on 60 amps. You get billed at the 150 amp usage. You could stage the startup for one machine at a time and you may want to keep them on to avoid spikes in usage. Installing VFD drives for each machine would also help. A variable frequency drive would allow you to slowly start each motor and avoid the spikes in usage created by hard starting machines.

From contributor L:
According to what I have been told, starting up the machines at the same time would make little difference in the demand rate, as it is averaged out in 15 minute intervals. Could be the way my power company does it and not yours, though.

From contributor B:
Unless your power company does not use demand meters on single phase (most likely they do), the demand charge should not be any different with power from the grid or with the use of rotary converters.

As others have said, it is not the momentary startup surge that spikes your demand charge, but rather the average current draw over a given period of time (15 minutes I believe here in CT).

Rotary converters are not magic wizards that make free electricity. It takes single phase 220v to spin the phase converter motor. That is being drawn from the grid. The 3rd leg being generated could be considered free, except for the fact that you are paying to spin the converter.

You also have the two legs from the grid going to each 3-phase motor. So since you are now spinning your machine motor with the single phase portion of the power system, and spinning the rotary converter motor, you are actually going to be using more power than if you were using straight 3-phase from the grid, at least theoretically.

To counter this whole argument, though, I'll add that I've had one or two people tell me their power usage and electric costs went down when they were using a rotary converter. They felt it had something to do with the rotary converter back feeding into the grid. Personally I don't see how this is possible.

From contributor C:
Your friend who thinks he can take single phase power, create 3 phase with a converter, and sell it back to the power company at a profit, needs to call me - I want a piece of that action! Sounds better than cold fusion!

From contributor K:
I can tell you that the amperage spike on a single phase service would be huge when running a phase converter. We don't have 3 phase available in our area and have to use a 60hp phase converter. Not only does the converter draw 70 amps constant, but when we fire up the wide belt, the amp spike is at least 250+ amps, not to mention the air compressor, which goes on and off several times a day.

Our single phase electric bill is anywhere from 1,500 to 2,000 a month and we are only a 3 man shop. If I could get 3 phase power here I definitely would.

From contributor R:
A rotary converter will show that it is pulling more amps due to bad power factor. This means that as the converter is spinning, it will be using power when the voltage is lower, as with AC power the voltage is on a sine wave, goes from high positive, to zero, then high negative. If the converter was spinning in harmony with the sine wave, the amps would be lower. This really does not matter in terms of power bills, as we are billed for watt hours, and amps are not really a factor in this. If you had a meter that corrected for power factor, you would see a much lower amp reading or watt reading. No matter if you have single or 3-phase power, you are being charged for watt hours, not amps. Demand meters are typically used for 3-phase as the power company will charge you for the max demand. They justify this by saying it is a fee to pay for the construction of the power plant, wires, etc., as they have to size their generating equipment based upon the max load that is used. The watt hour charge is for the actual electricity that is used.

On a commercial 3-phase service, the power is often cheaper per watt hour than a residential service. This is often due to a higher rate paid. As you use more electricity, you pay a penalty. In a commercial shop they want to sell more power to you and often the higher rates are not applied for higher use.

In terms of feeding power back to the grid, this is correct with some converters as when the motor slows it is using regenerative braking. This power needs to go someplace and it will go back into the grid. Much like how electric cars charge their batteries when slowing down. I am not sure if most meters will show the power going back into the grid. Obviously you had to use power to get the motor started and running, so it is not really generating a net gain as if you used a solar panel, but the power from the motor slowing will have to go some place (the motor does need to use a dynamic brake).

I use a digital phase converter which uses about 750 watts of power when idle, far less than a rotary converter. My machines do not know the difference between my generated 3-phase or power delivered by the utility. All of my phases are within a few volts of each other. The converter corrects for power factor errors, so the amp readings are much lower. I used to use a rotary converter and had issues starting a large shaper; it just did not have the backbone to generate the power. The digital converter runs my CNC machine perfectly. Since I am pulling less amps, the wires do not have to be as large as with a rotary converter.

From contributor B:
Good info. Thanks. It's interesting you mention that you went from a rotary to static converter due to machine startup issues. One would think it would have been the other way around.

When you say "digital" converter, I assume you are referring to a static converter. It is my understanding that these send a surge to the motor to get it started, but then the motor runs on single phase 220v. This reduces the power output of the motor by approximately 1/3. There is a lot of misinformation out there on this topic so I cannot state for 100% certain that this is the case.

I ran my 15hp Timesaver on a 10hp rotary converter quite successfully for years. Never a problem starting or running, as long as I didn't overload the sanding capacity of the machine.

I was always told that the generated 3rd leg from a rotary converter needed to be on one specific leg of a machine. I don't know for sure why this is the case, but I theorize that it's because you wouldn't want the generated leg on the coils and electronics of the machine circuitry. Perhaps your machine gave startup problems because of this? Did you pay attention to which input leg on the machine had the rotary converter generated 3rd leg?

From contributor R:
I should have been more clear; I am using a Phase Perfect converter, which does generate a 3rd phase of power. Any phase converter simply generates one of the phases and you use the other 2 that are supplied by the utility. A static converter provides the 3rd phase for a short period, long enough for the motor to start, then shuts off. It is often capacitors, with a delay built in, that give a burst of power. You are correct that the motor only uses 2/3 of the power and must be derated. On a CNC machine you cannot use this, as the motors are on frequency drives (variable speed) and need all 3 phases of power to work.

With the Phase Perfect all the legs are the same voltage, and it does not matter which leg is used for the electronics; there is never more than 1-2% difference in voltage. This is often better than 3-phase from a utility - as the legs are often not balanced due to single phase loads taken from 1 or 2 of the legs, thus resulting in lower voltage.

I had 2 Kay Phasemaster converters for running my shop equipment (before CNC) and could not get my Martin shaper into the second speed - it has a 2 speed motor and is very hard on the power to get to the second speed, almost like shifting a car from 1st to 4th gear. I ran a widebelt sander and other machines without issue on the rotary converters.

One of my main concerns with a rotary converter is that it is sized to a particular load, and if this load is not there, the voltage is very high from the generated phase. This is why rotary converters have a minimum motor size.

After changing to the Phase Perfect, my machines started faster and quieter. I have a 30 hp model.

From contributor U:
I just went through this mess you are describing, except I'm in a rural area and do not have 3-phase available. The 60 hp rotary phase converter is working but we have gotten some alarms on our CNC router related to this. The electric company said you would always be better off to use the power company's 3-phase when available. I was on a residential meter with a 15 KVA transformer and I had to upgrade to a 100 KVA. The electric company had to change my service to a 400 amp, which puts it in a CT or demand metering charge. The way they explained it was that it averages the last three 5 minute blocks every 15 minutes, it continues to average the last 15 minutes by dropping the first 5 minute block and picking up the new 5 minute block. So it really is averaging every 5 minutes.

I had the power company come out and try to help figure out how to beat the demand meter (I even tried to get another meter set), and the only way you can help it is to turn large motors on, and let them come up to speed before turning the next one on. My electric bill is about $900.00, and right at half of that is the demand charge. If I had 3 phase available, I would not have the big start up load with the 60 hp converter, so that would reduce the bill some.

They also explained that the demand charge is because I do not use a consistent amount of electric. If I used a small amount or a large amount all the time, I would be a better customer for them, but when you hit them with a large load for a little bit and then it tapers off, they have to purchase that power on the spot market, which is a lot more expensive for them.

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